This invention relates in general to a power take-off unit including a hydraulically actuated clutch assembly for selectively driving an accessory from a source of rotational power, such as a vehicle engine or transmission. More specifically, this invention relates to an improved structure for a power take-off unit including a hydraulically actuated brake assembly which is automatically engaged when the clutch assembly is disengaged, and further which is automatically disengaged when the clutch assembly is engaged.
Power take-off units are well known mechanical devices which are commonly used in conjunction with sources of rotational power, such as vehicle engines and transmissions, for rotatably driving a driven accessory. For example, power take-off units are commonly used in a variety of industrial and agricultural vehicles for operating hydraulic pumps which, in turn, operate hydraulically driven devices, such as plows, trash compactors, lifting mechanisms, winches, and the like. A typical power take-off unit includes a housing which rotatably supports an input gear (which is rotatably driven by the vehicle engine), an output shaft (which is connected to the driven device), and a set of meshing intermediate gears. The meshing intermediate gears are connected in a gear train between input gear and the output shaft so as to provide a rotatable driving connection between the engine of the vehicle and the driven accessory. The set of gears permits one or more speed reduction gear ratios to be provided between the input gear and the output shaft.
In many instances, the power take-off unit further includes a clutch assembly for selectively disconnecting the output shaft from the input gear. The use of a clutch assembly in a power take-off unit is desirable because it permits selective or intermittent operation of the driven accessory without having to turn off the vehicle engine. When the clutch assembly is engaged, the output shaft is rotatably driven by the input gear. Consequently, the driven device is operated by the vehicle engine. Conversely, when the clutch assembly is disengaged, the output shaft is not rotatably driven by the input gear. As a result, the driven device is not operated by the vehicle engine.
During operation, the meshing of the various gears contained within the housing of the power take-off unit can generate significant amounts of undesirable friction and heat. To minimize the adverse effects of this friction and heat, it is common to provide a quantity of lubricating fluid within the housing of the power take-off unit. When the input gear of the power take-off unit is connected to a vehicle transmission, for example, the lubricating fluid may be the transmission lubricating fluid. The teeth of the various gears contained within the housing of the power take-off unit are moved through the lubricating fluid during operation to minimize the adverse effects of friction and heat.
As mentioned above, when the clutch assembly is disengaged, the output shaft is not rotatably driven by the input gear, and the driven device is not operated by the vehicle engine. In this disengaged condition, the output shaft is simply disconnected from the input gear and is allowed to free-wheel within the housing of the power take-off unit. Under most conditions, the combination of being disconnected from the source of rotational power with the inherent resistance to further movement imposed by the load of the driven device causes the output shaft to immediately cease further rotation until the clutch assembly is subsequently re-engaged. Unfortunately, it has been found that under certain circumstances, the relatively viscous nature of the lubricating fluid can cause the output shaft to continue to be at least partially rotatably driven, even though the clutch assembly has been disengaged. This undesirable rotation of the output shaft after disengagement of the clutch assembly can, for example, occur when the temperature of the lubricating fluid is relatively cold and viscous, such as when the vehicle is initially started. Obviously, rotation of the output shaft of the power take-off unit (and, thus, operation of the driven device) after the clutch assembly has been disengaged is undesirable.
To address this, it is known to provide a power take-off unit with a brake assembly to slow or stop rotation of the output shaft. In one known power take-off unit, an internal drag brake constantly applies a braking force to the output shaft to retard rotation thereof, regardless of whether the clutch assembly of the power take-off unit clutch is engaged for operation or disengaged for non-operation. This constantly-braked power take-off structure is undesirable because it requires frequent adjustments to insure that the proper amount of braking force is applied to the output shaft. This is because the various components of the brake assembly will wear during use, thus varying the amount of the braking force. Also, wear and heat occur rapidly and frequently within the power take-off unit, resulting in frequent maintenance and repair. Furthermore, the magnitude of the braking force which can be exerted is limited so as to not adversely affect the operation of the power take-off unit when the clutch assembly is engaged for normal use.
In another known power take-off unit, a braking member is mechanically is actuated by the clutch assembly of the power take-off unit. In this mechanically actuated power take-off structure, a clutch piston of the clutch assembly is urged toward a disengaged position by a spring. To engage the clutch assembly, hydraulic fluid is applied to move the clutch piston against the urging of the spring to an engaged position. The braking member is connected to the clutch piston through a mechanical linkage. Thus, when the clutch piston is in the disengaged position, the braking member is in the engaged position. Conversely, when the clutch piston is in the engaged position, the braking member is in the disengaged position. This mechanically actuated brake assembly structure has been found to be undesirable because the brake assembly cannot be operated independent of the clutch assembly. Rather, operation of the brake assembly is totally reliant on the successful operation of the clutch assembly. Thus, it would be desirable to provide an improved structure for a power take-off unit including an internal brake assembly which avoids the drawbacks of these known structures.